1. Field of the Invention
The present invention relates the fixing of toner images to copy paper, and more particularly to the fixing of such toner images by the roller driven devices.
2. Description of the Related Art
An apparatus comprising a drum coated with a light-sensitive material and a fixing device is normally employed in a copying system or the like. To make a copy, a toner image is formed on the drum, which is then transferred to a paper. Then, the paper with the toner image is passed through the fixing device to fix the toner image to the paper.
In the conventional copying system as shown in FIG. 11, a document is placed on the body of the system. An optical reader projects lights to the document to read its content. Light reflected from the document illuminate on the drum 1 to form a latent image on the drum 1. The surface of the drum 1 has been electrostatically pre-charged with corotrons by corona-charging wires 2.
A developing device 3, which is disposed near the illuminated region on the drum 1, applies a toner to the electrostatically charged latent image on the drum and transforms the latent image into a visible toner image. The resulting toner image is then transferred to a paper P as corotrons are discharged from the drum 1 to the paper P by corona discharging wires 4.
The paper P with the transferred toner image is peeled off from the drum 1 and then transported to an entrance guide plate 6 by a drive mechanism 5 and then nipped, sometimes referred to hereinafter as gripped, in a fixing device 10. The fixing device 10 includes a heating roller 11 and a pressure roller 15. The rollers 11 and 15 apply simultaneously a heat and pressure to the paper P to fuse the toner of the toner image on the paper onto the paper while the paper is gripped between the rollers 11 and 15. The toner image consequently becomes permanently fixed to the paper. Then, the paper is fed through a delivery rollers 7.
In the conventional copying system, the fixing device is usually of a heating-roller type. The heating-roller type fixing device typically includes a hot heating roller and a cold pressure roller: The heating and pressure rollers simultaneously apply a heat and pressure to the paper, respectively, as the paper is nipped between the two rollers. Tungsten halogen lamps are often used as a heating source in the heating roller.
The heating-roller type fixing device is known to be safer, more cost effective, and energy efficient than other types of fixing devices. Therefore, it is most often used in the copying system and the like. The heating-roller type fixing device is disclosed in Japanese Patent Laid-Open Nos. 138274/1986 and 123877/1985, and elsewhere. In the heating roller type fixing devices previously disclosed, only one of the two rollers in the fixing device is driven, to cause the other roller to follow the driven roller. In other words, only one roller is driven to drive both rollers.
The mechanism of a typical heating-roller fixing device is shown in FIG. 12. A fixing device 10 includes a heating roller 11 and a pressure roller 15. The heating roller 11 is coated with a hard material like fluorocarbon resin or rubber. The pressure roller 15 is coated with a softer elastic material. A paper P carrying a toner image is nipped between the two rollers. Lamps 12 are incorporated in the heating roller 11 to maintain the heating roller at a constant temperature around 160.degree. to 220.degree. C. The heating roller 11 makes contact with the toner image side of the paper P. A heat and pressure is simultaneously applied to the paper from the heating and pressure rollers respectively, to fuse the toner image on the paper and permanently fix it to the paper.
A chain 14 is entrained with a sprocket mounted on the shaft of the heating roller 11 and a sprocket on a driving device 13 to drive the heating roller from the driven device 13. The pressure roller 15 having an arm 16 held to its shaft is disposed corresponding to the position of the heating roller 11. One end of the arm 16 is pivotally mounted to a pivot 17, and the other end is held by a spring 18. The spring 18 provides a given nipping pressure between the two rollers.
In the conventional heating-roller type fixing device, the pressure roller 15 follows the rotation of the heating roller 11 and rotates at the same peripheral velocity as the heating roller The conventional fixing device works well when small size papers like A2 or A4 size are used: the paper does not ruck up or wrinkle, and the toner image becomes satisfactorily fixed to the paper. However, when larger size papers like A0 or A1 size are used, the paper rucks and the toner image does not become satisfactorily fixed to the paper.
This situation is graphically illustrated in FIG. 13, in that the point at which the fixing characteristic curve A and the ruck characteristic curve B intersect is positioned well below a target value. Therefore, to avoid paper rucking or wrinkling for large size papers in the conventional fixing device, the nipping or gripping pressure is often reduced to the extent that the quality of the toner image fixed to the paper deteriorates, thus making high print quality copying difficult.